Visible light surface emitting semiconductor laser

ABSTRACT

A vertical-cavity surface-emitting laser is disclosed comprising a laser cavity sandwiched between two distributed Bragg reflectors. The laser cavity comprises a pair of spacer layers surrounding one or more active, optically emitting quantum-well layers having a bandgap in the visible which serve as the active optically emitting material of the device. The thickness of the laser cavity is m λ/2n eff  where m is an integer, λ is the free-space wavelength of the laser radiation and n eff  is the effective index of refraction of the cavity. Electrical pumping of the laser is achieved by heavily doping the bottom mirror and substrate to one conductivity-type and heavily doping regions of the upper mirror with the opposite conductivity type to form a diode structure and applying a suitable voltage to the diode structure. Specific embodiments of the invention for generating red, green, and blue radiation are described.

FIELD OF THE INVENTION

This relates to semiconductor lasers and, in particular, to verticallyarranged heterostructure lasers that emit electromagnetic radiation inthe visible spectrum (i.e., in the range between 400 and 700nanometers).

BACKGROUND OF THE INVENTION

Conventional edge-emitting laser diodes are well known. In these diodes,laser radiation is emitted in a plane that is a continuation of theplane of the p-n junction that forms the diode. Different types of thesediodes are widely used to provide laser radiation in the infrared andvisible regions. While these diodes have enjoyed considerable commercialsuccess, they are relatively large and, as a result, are difficult tointegrate with other devices.

Recently, a new class of semiconductor lasers known as a vertical cavitysurface emitting laser has been developed. Unlike the edge-emittinglaser, these vertical cavity lasers emit laser radiation in thedirection perpendicular to the plane of the p-n junction formed in thelaser diode. Considerable information concerning the structure andformation of such laser diodes is set forth, for example, in U.S. Pat.No. 4,949,350; in J. Jewell et al., "Microlasers," Scientific American,Vol. 265, No. 5, pp. 86-94 (November 1991); in J. Jewell et al.,"Vertical-Cavity Surface-Emitting Lasers: Design, Growth Fabrication,Characterization," IEEE Journal of Quantum Electronics, Vol. 27, No. 6,pp.1332-1346 (June 1991); in G. R. Olbright et al., "Cascadable LaserLogic Devices: Discrete Integration of Phototransistors withSurface-Emitting Laser Diodes," Electronics Letters, Vol. 27, No. 3,pp.216-217 (Jan. 31, 1991); and in J. Jewell et al., "Vertical CavityLasers for Optical Interconnects," SPIE Vol. 1389 InternationalConference on Advances in Interconnection and Packaging, pp. 401-407(1990), all of which are incorporated herein by reference.

As set forth in certain of the above-referenced publications, verticalcavity lasers have numerous advantages over edge-emitting lasers, someof the most important of which are that they can be fabricated inextremely small sizes (e.g., on the order of one micrometer in diameter)and can readily be integrated with other devices such as transistors.

To date, however, applications of vertical cavity lasers have beenlimited by the absence of any vertical cavity laser which emits visiblelaser radiation.

SUMMARY OF THE INVENTION

We have invented vertical-cavity lasers which emit laser radiation inthe visible region. In these devices, laser radiation can be stimulatedby optical pumping or by electrical injection. In a preferred embodimentof the invention, a vertical-cavity surface-emitting laser comprises alaser cavity sandwiched between two distributed Bragg reflectors. Thelaser cavity comprises a pair of spacer layers surrounding one or moreactive, optically emitting quantum-well layers having a bandgap in thevisible which serve as the active optically emitting material of thedevice. The thickness of the laser cavity is mλ/2 n_(eff) where m is aninteger, λ is the wavelength of the laser radiation and n_(eff) is theeffective index of refraction of the cavity. Electrical pumping of thelaser is achieved by heavily doping the bottom mirror and substrate toone conductivity-type and heavily doping regions of the upper mirrorwith the opposite conductivity type to form a diode structure andapplying a suitable voltage to the diode structure.

Specific embodiments of the invention for generating red, green, andblue radiation are described.

BRIEF DESCRIPTION OF THE DRAWING

These and other objects, features and advantages of the invention willbe more readily apparent from the following detailed description of theinvention in which:

FIG. 1 is a cross-section of a portion of a preferred embodiment of asurface emitting laser of the present invention;

FIGS. 2-7 are cross-sections of different portions of the surfaceemitting laser of FIG. 1; and

FIG. 8 is a top view of a portion of the surface emitting laser of FIG.1.

DETAILED DESCRIPTION

As shown in FIG. 1, the surface emitting laser of the present inventioncomprises a first mirror layer 10, a first spacer layer 20, a quantumwell layer 30, a second spacer layer 40 and a second mirror layer 50.Following techniques known in the art and described, for example, in theabove-referenced U.S. Pat. No. 4,949,350, layers 10, 20, 30, and 40 anda portion of layer 50 are epitaxially formed on a substrate 60. Theremaining portion of layer 50 is formed by dielectric deposition. As aresult, layers 10, 20, 30, 40, and 50 have the same diameter assubstrate 60. After epitaxial formation of the layers, well layer 30,spacer layer 40, and mirror layer 50 are defined by optical lithographyand etching to form a plurality of columns 70. Electrical contacts tosecond mirror layer 50 and substrate 60 are provided at 56 and 66.

As shown in the top view of FIG. 8, electrical contact 56 is preferablya bonding pad surmounting a portion of column 70. In this case, eachcolumn 70 has a first portion 71 (also shown in FIG. 1) that isapproximately 20 micrometers on a side and a second portion 72underneath the bonding pad that is approximately 100 micrometers on aside. Illustratively, substrate 60 has a diameter of 3 or 4 inches (7.5or 10 cm.) during epitaxial processing and is diced into several unitsfor use.

In a preferred embodiment of the invention which is used to generate redlight, substrate 60 is n+ doped GaAs and each of layers 10, 20, 30, 40,and 50 comprises a plurality of layers, the composition of which isillustrated in FIGS. 2-7 and described in more detail below.

As shown in FIG. 2, mirror layer 10 comprises alternating layers 11, 12of n+ doped AlAs and AlGaAs. Each layer is a quarter-wavelength thickwhere the wavelength is the wavelength in the layer of the radiationbeing emitted by the laser. As will be recognized by those skilled inthe art, the construction of mirror layer 10 is that of a distributedBragg reflector in which the AlAs is the layer having the lower index ofrefraction and AlGaAs is the layer having the higher index ofrefraction. As is also known, the index of refraction of AlAs isapproximately 3.0 depending on the wavelength, and the index ofrefraction of AlGaAs ranges from approximately 3.0 to 3.6 depending onthe wavelength and the relative concentrations of Al and Ga.

As shown in FIG. 3, the laser cavity comprises spacer layer 20, quantumwell layer 30, and spacer layer 40. The length of the laser cavity(which is the thickness of layers 20, 30, and 40) is mλ /2 n_(eff) whereλ is the free space wavelength of laser radiation emitted, m is aninteger and n_(eff) is the effective refractive index of the cavity.Advantageously, an active quantum well 34 is defined by an annular zone33 of implanted protons which surrounds the active quantum well therebyconfining the electrical current flow to the active QW. Details of theuse of ion implantation for such current funnelling are set forth in Y.H. Lee, et al., "Top-Surface-Emitting GaAs Four-Quantum-Well LasersEmitting at 0.85 μm," Electron Lett., Vol. 26, pp. 1308-1310 (1990) andH.-J. Yoo, et al., "Low Series Resistance Vertical-CavityFront-Surface-Emitting Laser Diode," Appl. Phys. Lett., Vol. 56, No. 20,pp. 1942-1945 (May 14, 1990), which are incorporated herein byreference.

As shown in FIG. 4, spacer layer 20 comprises a plurality of layers ofAlGaInP with progressively increasing amounts of Ga toward the quantumwell layer. As is known in the art, these layers are lattice matched toGaAs. Also as is known, the index of refraction of these layersincreases with increasing amounts of Ga and the bandgap decreases. Asshown in FIG. 6, spacer layer 40 is similar in construction withprogressively decreasing amounts of Ga toward mirror layer 50.

As shown in FIG. 5, quantum well layer 30 comprises three approximately50 Angstrom thick layers 31 of GaInP separated by two approximately 90Angstrom thick barrier layers 32 of AlGaInP. A peripheral zone 33 ofprotons is formed in these layers by implantation. This zone limits theactive quantum well to those portions 34 of layers 31 where protons arenot implanted. Peripheral zone 33 also confines current flow through thelaser diode to the portions of layers 31 and 32 where protons are notimplanted and, therefore, increases current density through the quantumwell.

As shown in FIG. 7, second mirror layer 50 comprises a plurality ofalternating layers 51, 52 of p+doped AlAs and AlGaAs, a peripheralelectrical contact layer 53 of Au and a plurality of alternating layersof 54, 55 of TiO₂ and SiO₂. Again, each of layers 51 and 52 and layers54 and 55 are a quarter-wavelength thick and these layers constitute adistributed Bragg reflector. The reflector, however, is partiallytransmissive to provide for emission of laser radiation from theuppermost layer. Advantageously, the center hole in contact layer 53 isdimensioned so as to restrict the propagation modes of the emitted laserradiation to the TEM₀₀ mode.

Preferably, as shown in the top-view of FIG. 8, electrical contact withcontact layer 53 is made in conventional fashion through a bonding pad56 which is formed at the same time as layer 53 on a portion 72 of theepitaxial layers immediately adjacent to the portion 71 of those layersillustrated in FIGS. 2-7.

If desired, the construction of second mirror layer 50 could be the sameas that of layer 10. However, the layers of AlAs and AlGaAs haverelatively high resistance which results in unwanted heating of thesecond mirror layer. Accordingly, we have found it advantageous toreduce the resistance of the second mirror layer by including only a fewlayers of 51, 52 of the mirror within the diode region betweenelectrical contact layer 53 and substrate 60. The remaining layers ofthe second mirror are formed by dielectric deposition of alternatingSiO₂ and TiO₂ layers on top of contact layer 53.

Individual lasers are formed by defining the devicesphotolithographically and etching them using known gaseous or chemicaletchants.

With appropriate selection of materials and layer dimensions, the laserof FIGS. 1-8 can be used to generate laser radiation in differentportions of the visible region of spectrum. The specific embodimentdescribed in conjunction with FIGS. 1-8 can be used to generateradiation in the red region.

To generate radiation in the yellow to green portion of the spectrum,the active quantum well layers preferably should be made of AlGaP andthe barrier layers of AlGaP where different compositions of Al and Gaare used for the quantum well layers and barriers. The second mirrorshould be made of alternating layers of AlGaP and AlP.

To generate radiation in the blue portion of the spectrum, the activequantum well layers preferably should be made of AlGaN and the barrierlayers of AlGaN where different compositions of Al and Ga are used forthe quantum well layers and barriers. The second mirror should be madeof alternating layers of AlGaN and AlN.

Numerous variations in the invention will be apparent to those skilledin the art from the foregoing description. For example, other materialcombinations from the III-V and II-VI semiconductor groups such asZnCdSe can be used in place of the materials specified for the quantumwell layers, spacer layers and mirror layers.

What is claimed is:
 1. A vertical cavity, surface emitting laser that emits radiation having a wavelength in the range between 600 and 700 nanometers comprising:a substrate, a first mirror comprising a plurality of layers formed on said substrate, each layer having a thickness of λ/4 n where n is the index of refraction of the layer, a first spacer, formed on said first mirror, an active layer comprising at least one quantum well layer of GaInP formed on said first spacer, a second spacer formed on said active layer, and a second mirror comprising a plurality of layers formed on said second spacer, each layer having a thickness of λ/4 n, said first and second mirrors defining therebetween a laser cavity having a length equal to mλ/2 n_(eff) where m is an integer and n_(eff) is the effective index of refraction of the laser cavity, and said first and said second mirrors are of alternating layers of AlGaAs and AlAs, said second spacer and said second mirror being made of materials that are transmissive to radiation having a wavelength of λ/n.
 2. The laser of claim 1 wherein at least the first mirror and the second mirror are doped with dopants of opposite conductivity type to form a diode structure and the laser further comprises first and second electrodes for applying a biasing voltage to the diode structure.
 3. The laser of claim 2 wherein said second mirror has fewer layers than said first mirror of alternating layers of AlGaAs and AlAs between said substrate and one of sad electrodes thereby reducing the resistance of said second mirror.
 4. The laser of claim 1 wherein the laser is optically pumped by radiation having a wavelength that is shorter than radiation emitted by the laser. 